docs: def get_prepopulated_value(field, instance):
      """
      Returns preliminary value based on `populate_from`.
      """
      if hasattr(field.populate_from, '__call__'):
          # AutoSlugField(populate_from=lambda instance: ...)
          return field.populate_from(instance)
      else:
          # AutoSlugField(populate_from='foo')
          attr = getattr(instance, field.populate_from)
          return callable(attr) and attr() or attr